Such paper machine covers which revolve continuously on rollers usually concern dryer cloths for example which are used to dehumidify the paper web, especially by the supply of heat. Heat permeability, resistance to wear and tear, the ability to permeate and entrain air, as well as the surface structure are the essential features of such dryer cloths in order to achieve perfect quality of the paper web to be dried.
It is generally known to design such dryer cloths, which usually concern flexible planar objects, as fabrics, knitted fabrics, spiralized goods or support segments which are flexibly connected with one another and are usually produced as injection-molded parts. The most frequently used ones are woven dryer cloths, followed by such in form of spiralized goods.
In the two aforementioned types of dryer cloths the soiling of the contact surface of the cloth which faces the paper has proven most recently to be a particularly problematic point. One reason for the increasing soiling problem in the dryer section of paper machines is the increasing use of waste paper in paper production. Whereas the waste paper share in Germany is on the average 60% for example, waste paper shares of 100% are reached occasionally, which leads to particular problems with the soiling of the dryer cloths. The type and composition of xe2x80x9csoilingxe2x80x9d are numerous: Resins, oils, greases, tar, so-called hot melts, starch, adhesive impurities (so-called xe2x80x9cstickiesxe2x80x9d) or plastic binders (so-called xe2x80x9cwhite pitchxe2x80x9d), which may be present in combined form under certain circumstances, contribute to the soiling. The types of soiling may occur in solid, adhesive or dissolved form. Usually, the size of the dirt particles is below 150 xcexcm.
Deposits of dirt particles on dryer cloths pose a problem because they impair relevant cloth properties such as the air permeability and the ability to entrain air, as well as paper contact and the heat permeability. This obstructs the even drying and perfect transport of the paper web. Moreover, the energy demand required for drying rises and the service life of the cloths decreases. Whereas deposits adhering to the contact surface of the cloth may cause an uneven humidity profile and possible detachments of the paper web, the deposits produce holes or thin places in the paper web after their detachment from the cloth which have a negative effect on the later printability of the paper.
Further developments of dryer cloths were made in the past with the predominant goal to increase the share of the contact surface in the overall surface area of the cloth in order to improve both the drying as well as the transport properties in this way. Such a goal orientation of the development can be noticed especially in the construction of dryer cloths for fast-running paper machines in particular. In one form of fabric construction the contact surface is defined as the sum total of the numerous individual surfaces of individual cloth wires which come into contact with the paper web.
The contact surface could be increased to a substantial extent with the introduction of long-floating cloth designs as compared with cloth fabric designs with simple skeining longitudinal wires. Usually, round or rectangular wire cross sections were processed. A further substantial enlargement of the contact surface could be achieved by the use of so-called flat strips as longitudinal wires, i.e. in the direction of the running direction of the machine. Wires are called flat strips whose ratio of width to thickness is substantially larger (e.g. 3:1) than in conventional flat wires. With such cloth designs based on the flat strip technology it is possible to achieve contact surfaces of close to 60%. In contrast to the cloths made of round or rectangular wires, such cloth designs offer a contact surface which is composed of fewer but larger contact areas instead of such with more, but smaller contact areas.
If possible, a cloth cleaning system is installed in the areas of the dryer section which are at the front as seen in the running direction of the machine, which cloth cleaning system may come with a continuous or periodic operating mode. In this part of the dryer section the paper web still has a relatively high humidity content and is therefore particularly sensitive with respect to the entrainment of dirt particles which detach from the contact side of the dryer cloth.
Especially in cases in which no (efficient) cleaning system can be installed in said front sections of the dryer section relatively large dirt particles can form, which depends on the composition of the paper material, the process conditions and the type of dirt particles. As such, they can detach from the cloth surface, whereby they cause quality impairments in the paper due to their size.
The invention is based on the object of providing a cover for a paper machine in which the inclination towards the adherence of dirt particles is reduced. Furthermore, the size from which the agglomerated dirt particles detach from the contact surface is to be reduced.
This object is achieved in accordance with the invention by a cover in which at least the surface facing the paper web of at least one part of the elements forming the contact surface comprises at least partially an averaged surface roughness of between 5 xcexcm and 100 xcexcm. The determination of the averaged surface roughness is carried out based on the DIN EN ISO 4287. If the adherence to the definitions as contained therein concerning the measured length is not possible because a measurement transversally to the longitudinal direction of a narrow tape, it is possible to use alternatively a contactless laser measurement.
The invention is based on the finding that a cover with a surface roughness of the contact surfaces in the aforementioned range does not comprise any concatenated larger contact areas as a result of the limited flexibility of the paper web, which contact areas could be used by the dirt particles as adhering surfaces. In contrast to previously known covers, growing agglomerations of dirt particles will detach from the cover in accordance with the invention before they can reach a size critical for leading to quality problems in the paper web. The surface areas facing the paper web of the elements forming the contact surface in covers according to the state of the art usually comprise an averaged surface roughness in the range of between approx. 1.5 and 3.0 xcexcm. This surface roughness is thus substantially lower than the one proposed in accordance with the invention and is obtained especially from the usual production methods for the elements forming the contact surfaces, namely extrusion in wires or strips or injection molding for flexibly connected support segments. In said production processes the elements are realized with the lowest possible viable surface roughness for economic reasons in order to meet the common assumption that the smoothest possible surface should lead to the lowest possible inclination towards soiling.
In contrast to this, it was recognized with the present invention that in view of the xe2x80x9cfirst-order surface structurexe2x80x9d (coarse structure) as achieved for example by the type of fabric and the thread dimensions used therein (wires or strips) and the type of binding, the realization of the largest possible individual surfaces may be useful. The same applies for example to the dryer cloths made of injection molded segments. At least the contact surfaces of said first-order surface structures should be provided in accordance with the invention with an additional xe2x80x9csecond-order surface structurexe2x80x9d (fine structure) which lies in the range of the aforementioned averaged surface roughness. In cooperation with the given flexibility of the paper web to be conveyed, such a surface provided with the fine structure acts as a virtually plane contact surface with the advantage that markings on the paper web can hardly be caused by said fine structure. Due to the planarly reduced individual contact areas, the fine structure produces a clearly improved possibility for detachment especially for dirt particles produced by agglomeration, so that the same will detach at a considerably earlier time, i.e. with a substantially smaller size, and will therefore not lead to the known decreases in quality on the paper surface.
In a preferred embodiment of the invention the cover is a fabric, with at least a part of its longitudinal threads having an averaged surface roughness of between 5 xcexcm and 100 xcexcm at the surface facing the paper web.
Such an embodiment is preferable for production reasons because the fine structure of the cover can be realized by maintaining the usual weaving techniques and types of binding already during the production of the longitudinal threads for example, e.g. by way of extrusion.
In covers in which a substantial part of the contact surface is formed by threads extending transversally to the running direction of the machine it makes sense conversely that transversal threads of the fabric have an averaged surface roughness of between 5 xcexcm and 100 xcexcm at least in the surface facing the paper web.
In a further development of the invention it is provided that longitudinal and/or transversal threads of the fabric are profiled in the cross section. Especially in the case of producing the threads by way of extrusion, does this prove to be particularly uncomplicated from a production viewpoint.
As an alternative it is also possible that threads are profiled in a longitudinal section.
An also very appropriate embodiment of the invention is that the elements forming the contact surface towards the paper web are spirally extending threads whose surface facing the paper web comprises an averaged surface roughness of between 5 xcexcm and 100 xcexcm.
It is moreover within the scope of the invention that the elements forming the contact surface towards the paper web are injection-molded segments which are each connected flexibly with adjacent injection-molded elements at least in the longitudinal direction of the dryer cloth and whose surface facing the paper web comprises at least partly an averaged surface roughness of between 5 xcexcm and 100 xcexcm.
A particularly effective prevention of dirt particle accumulations and agglomerations occurs in the range of an averaged surface roughness of between 10 xcexcm and 80 xcexcm. Preferably, a surface roughness of between 30 xcexcm and 70 xcexcm should be realized.
A further development of the cover in accordance with the invention is that the elements forming the contact surface consist of two different materials.
This allows providing the layer facing the paper web with the surface roughness in accordance with the invention and, at the same time, to take material properties into account which lead to a particularly favorable paper conveyance and a lower tendency for the adherence of particles. By choosing suitable materials for a bottom layer averted from the paper web it is also possible at the same time to ensure the strength properties and the heat conductivity properties of the cover for example.
A particularly advantageous possibility for achieving a multi-layer element for forming the contact surface is that the surface facing the paper web of the elements forming the contact surface is produced by coating a basic body. In principle, both the coating of a finished cover such as a fabric or the coating of individual elements such as the threads or the injection-moulded elements composing the cover are possible.
Finally, it is provided for in accordance with the invention that the elements forming the contact surface are produced by way of a multi-component extrusion in order to ensure an intimate connection between the different materials of the elements.